Monday, December 16, 2013

Rumination on Electric Drivetrain for CarBEN EV5

The background on this is I was able to be at the X-Prize Knockout Round as a guest member of the Edison2 team, and while I was there I meet and talked to many of the people, and to see what they had built, and all the effort and passion that went into it. Jason Fagone has written a enthralling and detailed book about the X-Prize, called 'Ingenious' and he and Kevin Smith of Illuminati Motor Works are touring with the IMW 'Seven' electric car.

Seeing the IMW Seven and talking to Kevin Smith and to some of the folks at the MIT Electric Vehicle Team has got me thinking more about the main electric drivetrain for CarBEN EV5 - which is a 5 seat electric car that I am currently building. I've been concentrating on the design and how to construct it, so this is a first take on the drivetrain components:

I can fit 106 of these in CarBEN EV5 (Seven has 99) and Kevin Smith swears by them and their ruggedness. That would be ~ 35.3 kWh and cost roughly $16,000. They use no BMS and do a "bottom balance" and have stopped seeing stress that occasionally caused the cells to swell.

I had used the EiG 20Ah pouch/prismatic cells in my planning and I could probably fit about 50 kWh into the compartments in the floor. FVT has an innovative BMS system that shunts the excess from one cell to another cell that is still charging; rather than to ground - as I understand it. This should be more efficient than ordinary BMS.

I looked briefly at the A123 cells, and these are wider than would easily fit in the battery compartment I have in CarBEN. And they are very hard to find, if not impossible.

A choice for charger is still vague. Seven has a Manzanita, and it works well. They disconnect it on the inside, so that regen doesn't blow it out.

One of the MIT EVT people has put me in touch with their Porsche guy, and I have asked about getting the rear suspension from a 914. He is a busy person (post grad?) and I will wait and see if he can help me. Up until now, I've been thinking of using the front suspension of a Saab 900. But I'm also going to look at the Neon front suspension that Seven uses (in the front and back), though I don't think it could fit in the back of CarBEN, and I don't need the Prelude rear wheel steering; as the wheelbase of CarBEN is significantly shorter.

It looks like all this hardware will cost $35-40K, which is actually about what I was expecting.

The Illuminati Seven sets a high standard for efficiency: it uses just 129Wh/mile at ~60mph, and it has a 220+ mile range at 60-70mph. The EPA rating (tested by Chrysler at their proving grounds in Chelsea MI) for 7 is 207MPGe.

8 comments:

Neil it was a pleasure to meet you at MIT and I'm very impressed by all the work you have put into carBEN.

I'm a mechanical engineer and love that you are digging in and designing a better mouse trap. But I have a personal philosophical dilemma with the effort. As much as I'm all for this effort, the prospect of extracting $15K out of my own pocket for the electric motor alone would stop me cold. Of course I work on projects all the time where I'm spending this kind of money buying electric motors, but it is someone else's money, not my money.

I did spend $12K of my own money to build solar hot water panels, but got a 30% rebate from the government. Also, I think the payback on the hot water panels versus oil heat seems pretty reasonable. But paying more for a prototype car than it would cost to buy a Nissan LEAF...well...that seems to be a lot more than I could stomach.

I hear what you are saying, John. I am hoping actually to do a crowd funding campaign to buy the big ticket stuff for CarBEN. And I hope to learn from this and build another one better and for less money. And I hope to have others learn from what I've done and then go on to build their own.

The IMW 'Seven' was about $110,000 for the X-Prize car, and now more for the second gen body. And that is a good cost for what it is.

If CarBEN can go 300+ miles and it costs me $45K including the battery, that is still less than a Tesla. :-)

Hello, Neil Blanchard, I am student from India currently pursuing mechanical engineering and am doing a project on a solar electric road vehicle along with a team of 29 members.Could you please provide me with your email id. I was hoping to contact you and get some guidance on aerodynamics.I really liked your above stated design and coefficient of drag is astonishing.

Regards,Darshan Mehta Mechanical EngineerStructures and Bodywork Head at Solar.Mobil Manipal

I do not know any actual Cd's for my design, as I have not been able to do any testing, or even any CFD work on my model. But, I am following some designs I know are low drag, so hopefully it is low drag.

Neil dot Blanchard at Verizon dot Net

It turns out that the Saab 900 I bought for the front suspension, will possible also have workable rear suspension, that I can use in CarBEN.

Hello Neil,Why dont you design your own suspension ? It would require some time, analysis and some software skills.But it would be to your exact specified criteria.Also i have sent you an email regarding CFD.

I bought the Saab 900 for $300. And the suspension should be very close to what I need. I have so much else to do to build CarBEN, I am glad to be able to use some existing parts.

I'll look for your email. The main challenge for using CFD is my model is not solid - it has lots of holes; so while it worked for me to generate the CNC files to construct the foam core to the chassis, it won't work for CFD. Unless you have some way to close all the holes in the model.